We used spiny lobsters (Panulirus argus) in a discrimination learning procedure with aversive conditioning to examine their behavioral discrimination of adenosine-5'-monophosphate (AMP), betaine, L-cysteine and their binary mixtures. Our results show that spiny lobsters can clearly discriminate among binary mixtures and their components. Lobsters aversively conditioned to avoid responding to a binary mixture continued to respond to that mixture's components, and lobsters that were aversively conditioned to avoid responding to a compound tended to continue to respond to binary mixtures containing that compound. Thus, responses of conditioned lobsters to binary mixtures were not usually intermediate between the responses to the mixtures' components, which might be expected for response-matched compounds. This result might arise from any of several factors. First, it might result from mixture interactions in the peripheral olfactory system, if the responses of olfactory receptor neurons to one component of a binary mixture were suppressed by the other component, making the response to the mixture more similar to the suppressing component. Electrophysiological data from a population of 50 singly-recorded olfactory receptor neurons (Daniel and Derby, 1994) do not consistently support this idea. A second possible reason for the behavioral response to a binary mixture not being intermediate between the responses to its components involves higher order processing, such as mixture interactions generated in olfactory interneurons in the CNS (which is known to occur: Derby et al., 1985; Ache, 1989), configural learning or associative processing.